In a communication method by which communication parameters such as a subcarrier and a modulation method used for transmission and reception are determined based on estimation on the characteristics of a transmission line, it is important to precisely determine the communication parameters that are suitable for the characteristics of the transmission line in transmission. In particular, in a communication system having attenuation characteristics that deeply depend on the frequency (power line carrier communications having a power line as a communication medium, for example), it is effective to use a multi-carrier transmission line method using a subcarrier and a modulation method that are suitable for the characteristics of the transmission line
In a transmission line estimation method used in conventional communication systems, a transmission line is estimated periodically or when the number of retransmissions due to communication errors exceeds a specified value (considering that the characteristics of the transmission line are deteriorated) Then, based on the result of this estimation on the transmission line, new parameters are selected, and data is sent or received. This technique has been disclosed in, for example, JP2002-158675A.
However, in an environment in which the characteristics of the transmission line vary periodically, the communication parameters selected when estimating the transmission line often do not suit for the characteristics of the transmission line when sending data if a timing of sending data is not synchronized with the periodical variation of the characteristics of the transmission line. Thus, in the above-described conventional method, the maximum communication efficiency is not always obtained even when the transmission line is estimated.
Thus, as a countermeasure for this problem, a following method has been conventionally proposed.
First, the variation cycle of the characteristics of a transmission line is synchronized with the frame period of a communication system, and this variation cycle is divided into a plurality of sections. Next, within one frame period, the plurality of divided sections of the transmission line are continuously estimated section by section. Then, as a result of the transmission line estimation, communication parameters obtained in a section with the highest communication efficiency are selected and then communications are performed. FIG. 12 is a process sequence of this conventional method for estimating a transmission line.
However, in the conventional method shown in FIG. 12, there is the problem that the transmission line is estimated continuously, and thus requests to estimate the transmission line and their response messages occupy the transmission line and disturb communications of stream data, audio data, or other data that is supposed to be sent. Furthermore, in this conventional method, a time from the starting point of the frame period of the communication system to the starting time of a transmission line estimation section is different for each frame period. As a result, when the band is guaranteed, for example, with time sharing, not only is scheduling for transmission line estimation complicated, but also arises a case in which a scheduling condition cannot be satisfied.